This invention relates to a marine propulsion system, and more particularly to an exhaust discharge system for a marine propulsion system.
In a marine propulsion system, it is known to route exhaust gas through an internal exhaust discharge passage formed in the housing of the marine propulsion system. In an outboard motor, for example, the discharge system typically includes an exhaust passage provided in the lower unit of the motor which houses the drive shaft and the gearcase. A propeller is generally mounted to a propeller shaft disposed in the lower end of the lower unit, and it is desirable to provide an exhaust passage which discharges exhaust through the hub of the propeller. Such discharge of the exhaust provides quiet operation of the motor and enhances performance. A similar manner of discharging exhaust is used in connection with a stern drive system.
It has been found to be desirable to minimize the surface area of the submerged portion of the marine drive lower unit transverse to the direction of water flow therepast, in order to reduce drag caused by such submerged portion. The desirability of reducing the transverse surface area of the submerged portion of the lower unit housing results in a configuration of the lower portion of the gearcase which provides a restricted area for exhaust flow through the exhaust discharge passage formed therein. This restricted area of exhaust flow is normally located just prior to entrance of the exhaust into the propeller shaft housing. At certain operating conditions, such area of restricted exhaust flow can cause a bottleneck in the exhaust discharge system which is unable to accommodate the exhaust output produced by the engine. In this circumstance, the performance of the engine suffers.
It has also been found that, to increase boat performance, it is desirable to vent exhaust gas into the path of the propeller as it rotates. Such discharge of exhaust gas into the propeller path ventilates the propeller, thus allowing the engine to move quickly into the power band for relatively rapid acceleration and planing ability.
The present invention is designed to relieve the bottleneck in the exhaust discharge system and also to ventilate the propeller. In accordance with the invention, in an exhaust system including an internal exhaust passage leading to an exhaust passage in the hub of the propeller, a pair of exhaust outlet passages are disposed upstream of the propeller hub exhaust passage. The exhaust outlet passages are in communication with the internal exhaust passage, and are arranged so that one passage is provided on each side of the lower unit. The pair of exhaust outlet passages are preferably located so as to be in the upper half of the path of the propeller, to provide controlled ventilation of the propeller. The sides of the gearcase may be provided with a flared or ramped area forwardly of the exhaust outlet passages for deflecting water over the exhaust outlet passages as the lower unit moves forwardly through the water.
In another embodiment of the invention, a substantially flat rear face is provided on the lower unit. An exhaust outlet passage is formed in the rear face upstream of the propeller hub exhaust passage for providing exhaust relief. The substantially flat rear face of the lower unit preferably has a transverse dimension at least approximately that of the greatest transverse dimension of the lower unit, and the exhaust outlet passage is located so that exhaust is discharged into the upper hal of the path of the propeller. This construction also provides controlled ventilation of the propeller, and also discharges exhaust into the low pressure area created by the flat rear face of the lower unit as it moves forwardly in the water, thereby increasing performance.